Once the horse is tracking up (hind feet following in the hoof prints of the front) and starting to reach over his back and out and down through his neck we can consider attaching side reins.
Which type of side rein to use?
My preferred choice is for the plain leather with no elastic or donuts for the horses to bounce around with or lean into. With the plain reins, if the horse attempts to lean they give as soon as the horse gives. Because the elasticated / rubber ring type stretch, they continue to exert a momentary backward pull after the horse has yielded.
What is the correct length for the side reins?
The answer is to ask the horse. I see side reins as an addition to a more educated horse's training not as a means of teaching an outline or head set. Their purpose is to teach the horse to come out and meet the contact without fear, to trust the rein in the same way that they will trust the rider’s hand.
A rule of thumb method for finding the length is to stand in front of the horse (and you may need a helper the first time you do this) facing his head. Adjust him so that his head and neck are coming straight out of his shoulders (i.e. no looking to left or right). Although it is considered correct to fasten the outside rein before the inside one (as when a rider picks up the outside rein contact before the inside when riding), it is best to make sure that both side reins are adjusted before attaching them to the cavesson or bit so that the horse doesn’t have to wait with a lop-sided contact while you fiddle around sorting the other one out.
Their purpose is not to ‘pull the head’ in or to ‘set a head carriage’
Side reins need to be short enough to allow a connection of the rein aids to the hind legs, but long enough that the horse's nose can be slightly in front of the vertical in motion.
When the horse is moving in balance, the side reins should be slightly slack with the nose slightly in front of the vertical, and there should be contact through the lunge line.
When adjusting the length or leading the horse with the reins unclipped make sure that they do not dangle freely. They can get tangled around the horses legs or worse stepped on. When not in use clip them to the rings on the surcingle or dees on the front of the saddle.
Ideally, they should be adjusted throughout the session, too, lengthening for canter and any prolonged or early walk work. I believe they are best suited to work in trot where the horse requires less movement in the neck to balance himself.
Should they be of the same length or should the inside one be adjusted shorter?
This is a debate that will run and run. For basic lungeing, I prefer to have the reins equal length, but have had it pointed out by a French trainer who does use variable lengths that he lengthens the outside one; he doesn’t shorten the inside. There’s a huge difference between doing one and the other. This adjustment is for more advanced horses accustomed to working ’in position’. (fig. 1)
They are most effective when used in trot as there is not so much movement through the horse’s head and neck in this gait. At the start of a lunge session, I let my horses walk on both reins without the side reins attached before adjusting them loosely for their warm-up trots. They can then be shortened to the horse’s optimum work length for that time (be aware that this length can vary from day to day and within a session) and if lungeing a younger or unfit horse, I will give short walk breaks with the reins unclipped, so he can have a good stretch. A more advanced horse can have the reins attached for a short period of walk without causing too many problems.
It is possible to pervert any good principle if it is used for a purpose for which it was never intended.
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Classical Dressage Notebook